In modern communication systems, high order modulations like 16QAM (Quadrature Amplitude Modulation) and 64QAM are adopted. Downlink transmitted signals are generated by adding up multiple statistically independent signals (for different users and carriers), which lead to high peak to average power ratio (PAR).
In order to increase power amplifier efficiency and to better utilize DAC (Digital Analog Converter) dynamic range, many CFR algorithms are used to reduce PAR of transmitted signal in Radio Unit side. Noise shaping and peak cancellation are two CFR algorithms in digital IF (Intermediate Frequency) domain. The similarity of the two algorithms is to use cascaded clipping stages to reduce PAR gradually. Different clipping stages have different clipping thresholds in order to achieve an optimal EVM (Error Vector Magnitude) result. Different sets of clipping thresholds are needed for IF signals that have different PAR values. For example, simulation results show clipping thresholds for multi-stage Noise shaping should follow an arithmetic series in order to achieve the optimal result.
Currently, one set of clipping thresholds will be fixed and applied to respective clipping stages after setup of one configuration, which clipping thresholds are determined in assumption of fixed original PAR and target PAR values for signals to be transmitted. Hereinafter, original PAR refers to PAR of signals before clipping, i.e. PAR of signals to be input into clipping stages, while target PAR refers to a target PAR of signals after clipping, i.e. an expected PAR of clipped signals from clipping stages. Both original PAR and target PAR are defined at certain probability point of CCDF (Complementary Cumulative Distribution Function) curve. Generally, the original PAR and the target PAR are in dB and are defined at probability 10−4.
However, due to changes of factors like carriers, the number of users and modulation types (QPSK, 16QAM or 64QAM), said original PAR after carrier combination will change. In this case, problem will arise with fixed clipping thresholds, since fixed thresholds will not adapt to change of original PAR, and then such a solution will not be an optimal solution for the balance between PAR reduction and EVM.